arXiv subjects

Tomohiro Otsuka

Publications and source records attributed to Tomohiro Otsuka.

At least 37 records · Page 2Linked to original sources

arXiv2022

Formation of quantum dots in MoS2 with cryogenic Bi contacts and intrinsic Schottky barriers

The recent advancement in two-dimensional (2D) materials-based quantum confinement has provided an opportunity to investigate and manipulate electron transport for quantum applications. However, the issues of metal/semiconductor interface effects create a hurdle to realize the artificial fabrication of the quantum dot and study the quantum transport. Here, we utilize the strategy of employing the semimetal for Ohmic contacts with transition metal dichalcogenides especially, multilayer MoS2 and successfully fabricate the MoS2-Bi based FET devices. We observe the Ohmic behavior in the MoS2-Bi devices at cryogenic temperatures 4.2, 2.3 and 0.4 K. We also utilize intrinsic Schottky barriers formed at the interface between MoS2 and Au for the gate electrodes to form and control quantum dots. We observed Coulomb diamonds in MoS2 devices at cryogenic temperature. Our results of quantum transport in MoS2 could serve as a stepping stone for investigating novel quantum effects such as spin-valley coupling and the manipulation of qubit systems.

cond-mat.mes-hall↗

arXiv2021

Gate voltage dependence of noise distribution in radio-frequency reflectometry in gallium arsenide quantum dots

We investigate gate voltage dependence of electrical readout noise in high-speed rf reflectometry using gallium arsenide quantum dots. The fast Fourier transform spectrum from the real time measurement reflects build-in device noise and circuit noise including the resonator and the amplifier. We separate their noise spectral components by model analysis. Detail of gate voltage dependence of the flicker noise is investigated and compared to the charge sensor sensitivity. We point out that the dominant component of the readout noise changes by the measurement integration time.

cond-mat.mes-hall↗

arXiv2020

Formation of quantum dots in GaN/AlGaN FETs

GaN and the heterostructures are attractive in condensed matter science and applications for electronic devices. We measure the electron transport in GaN/AlGaN field-effect transistors (FETs) at cryogenic temperature. We observe formation of quantum dots in the conduction channel near the depletion of the 2-dimensional electron gas (2DEG). Multiple quantum dots are formed in the disordered potential induced by impurities in the FET conduction channel. We also measure the gate insulator dependence of the transport properties. These results can be utilized for the development of quantum dot devices utilizing GaN/AlGaN heterostructures and evaluation of the impurities in GaN/AlGaN FET channels.

cond-mat.mes-hall↗

arXiv2020

Spin orbit field in a physically defined p type MOS silicon double quantum dot

We experimentally and theoretically investigate the spin orbit (SO) field in a physically defined, p type metal oxide semiconductor double quantum dot in silicon. We measure the magnetic field dependence of the leakage current through the double dot in the Pauli spin blockade. A finite magnetic field lifts the blockade, with the lifting least effective when the external and SO fields are parallel. In this way, we find that the spin flip of a tunneling hole is due to a SO field pointing perpendicular to the double dot axis and almost fully out of the quantum well plane. We augment the measurements by a derivation of SO terms using group symmetric representations theory. It predicts that without in plane electric fields (a quantum well case), the SO field would be mostly within the plane, dominated by a sum of a Rashba and a Dresselhaus like term. We, therefore, interpret the observed SO field as originated in the electric fields with substantial in plane components.

cond-mat.mes-hall↗

arXiv2020

Coherence of a driven electron spin qubit actively decoupled from quasi-static noise

The coherence of electron spin qubits in semiconductor quantum dots suffers mostly from low-frequency noise. During the last decade, efforts have been devoted to mitigate such noise by material engineering, leading to substantial enhancement of the spin dephasing time for an idling qubit. However, the role of the environmental noise during spin manipulation, which determines the control fidelity, is less understood. We demonstrate an electron spin qubit whose coherence in the driven evolution is limited by high-frequency charge noise rather than the quasi-static noise inherent to any semiconductor device. We employed a feedback control technique to actively suppress the latter, demonstrating a $π$-flip gate fidelity as high as $99.04\pm 0.23\,\%$ in a gallium arsenide quantum dot. We show that the driven-evolution coherence is limited by the longitudinal noise at the Rabi frequency, whose spectrum resembles the $1/f$ noise observed in isotopically purified silicon qubits.

quant-ph↗

arXiv2019

Quantum nondemolition measurement of an electron spin qubit

Measurement of quantum systems inevitably involves disturbance in various forms. Within the limits imposed by quantum mechanics, however, one can design an "ideal" projective measurement that does not introduce a back action on the measured observable, known as a quantum nondemolition (QND) measurement. Here we demonstrate an all-electrical QND measurement of a single electron spin in a gate-defined quantum dot via an exchange-coupled ancilla qubit. The ancilla qubit, encoded in the singlet-triplet two-electron subspace, is entangled with the single spin and subsequently read out in a single shot projective measurement at a rate two orders of magnitude faster than the spin relaxation. The QND nature of the measurement protocol is evidenced by observing a monotonic increase of the readout fidelity over one hundred repetitive measurements against arbitrary input states. We extract information from the measurement record using the method of optimal inference, which is tolerant to the presence of the relaxation and dephasing. The QND measurement allows us to observe spontaneous spin flips (quantum jumps) in an isolated system with small disturbance. Combined with the high-fidelity control of spin qubits, these results pave the way for various measurement-based quantum state manipulations including quantum error correction protocols.

cond-mat.mes-hall↗

arXiv2018

Difference in charge and spin dynamics in a quantum dot-lead coupled system

We analyze time evolution of charge and spin states in a quantum dot coupled to an electric reservoir. Utilizing high-speed single-electron detection, we focus on dynamics induced by the first-order tunneling. We find that there is a difference between the spin and the charge relaxation: the former appears slower than the latter. The difference depends on the Fermi occupation factor and the spin relaxation becomes slower when the energy level of the quantum dot is lowered. We explain this behavior by a theory which includes the first-order tunneling processes. We conduct detailed comparison of the experiment and the theory with changing the energy of the quantum dot levels, and the theory can reproduce the experimental results.

cond-mat.mes-hall↗

arXiv2018

Coherent transfer of electron spin correlations assisted by dephasing noise

Quantum coherence of superposed states, especially of entangled states, is indispensable for many quantum technologies. However, it is vulnerable to environmental noises, posing a fundamental challenge in solid-state systems including spin qubits. Here we show a scheme of entanglement engineering where pure dephasing assists the generation of quantum entanglement at distant sites in a chain of electron spins confined in semiconductor quantum dots. One party of an entangled spin pair, prepared at a single site, is transferred to the next site and then adiabatically swapped with a third spin using a transition across a multi-level avoided crossing. This process is accelerated by the noise-induced dephasing through a variant of the quantum Zeno effect, without sacrificing the coherence of the entangled state. Our finding brings insight into the spin dynamics in open quantum systems coupled to noisy environments, opening an avenue to quantum state manipulation utilizing decoherence effects.

cond-mat.mes-hall↗

arXiv2018

Four single-spin Rabi oscillations in a quadruple quantum dot

Scaling up qubits is a necessary step to realize useful systems of quantum computation. Here we demonstrate coherent manipulations of four individual electron spins using a micro-magnet method in a quadruple quantum dot - the largest number of dots used for the single spin control in multiple quantum dots. We observe Rabi oscillations and electron spin resonance (ESR) for each dot and evaluate the spin-electric coupling of the four dots, and finally discuss practical approaches to independently address single spin control in multiple quantum dot systems containing even more quantum dots.

cond-mat.mes-hall↗

arXiv2017

Robust Single-Shot Spin Measurement with 99.5% Fidelity in a Quantum Dot Array

We demonstrate a new method for projective single-shot measurement of two electron spin states (singlet versus triplet) in an array of gate-defined lateral quantum dots in GaAs. The measurement has very high fidelity and is robust with respect to electric and magnetic fluctuations in the environment. It exploits a long-lived metastable charge state, which increases both the contrast and the duration of the charge signal distinguishing the two measurement outcomes. This method allows us to evaluate the charge measurement error and the spin-to-charge conversion error separately. We specify conditions under which this method can be used, and project its general applicability to scalable quantum dot arrays in GaAs or silicon.

cond-mat.mes-hall↗

arXiv2016

Fast probe of local electronic states in nanostructures utilizing a single-lead quantum dot

Transport measurements are powerful tools to probe electronic properties of solid-state materials. To access properties of local electronic states in nanostructures, such as local density of states, electronic distribution and so on, micro-probes utilizing artificial nanostructures have been invented to perform measurements in addition to those with conventional macroscopic electronic reservoirs. Here we demonstrate a new kind of micro-probe: a fast single-lead quantum dot probe, which utilizes a quantum dot coupled only to the target structure through a tunneling barrier and fast charge readout by RF reflectometry. The probe can directly access the local electronic states with wide bandwidth. The probe can also access more electronic states, not just those around the Fermi level, and the operations are robust against bias voltages and temperatures.

cond-mat.mes-hall↗

arXiv2016

Tunneling induced spin dynamics in a quantum dot-lead hybrid system

Semiconductor quantum dots (QDs) offer a platform to explore the physics of quantum electronics including spins. Electron spins in QDs are considered good candidates for quantum bits in quantum information processing, and spin control and readout have been established down to a single electron level. We use these techniques to explore spin dynamics in a hybrid system, namely a QD coupled to a two dimensional electronic reservoir. The proximity of the lead results in relaxation dynamics of both charge and spin, the mechanism of which is revealed by comparing the charge and spin signal. For example, higher order charge tunneling events can be monitored by observing the spin. We expect these results to stimulate further exploration of spin dynamics in QD-lead hybrid systems and expand the possibilities for controlled spin manipulations.

cond-mat.mes-hall↗

arXiv2016

Detection and control of charge states in a quintuple quantum dot

A semiconductor quintuple quantum dot with two charge sensors and an additional contact to the center dot from an electron reservoir is fabricated to demonstrate the concept of scalable architecture. This design enables formation of the five dots as confirmed by measurements of the charge states of the three nearest dots to the respective charge sensor. The gate performance of the measured stability diagram is well reproduced by a capacitance model.These results provide an important step towards realizing controllable large scale multiple quantum dot systems.

cond-mat.mes-hall↗

arXiv2016

Coherent electron-spin-resonance manipulation of three individual spins in a triple quantum dot

Quantum dot arrays provide a promising platform for quantum information processing. For universal quantum simulation and computation, one central issue is to demonstrate the exhaustive controllability of quantum states. Here, we report the addressable manipulation of three single electron spins in a triple quantum dot using a technique combining electron-spin-resonance and a micro-magnet. The micro-magnet makes the local Zeeman field difference between neighboring spins much larger than the nuclear field fluctuation, which ensures the addressable driving of electron-spin-resonance by shifting the resonance condition for each spin. We observe distinct coherent Rabi oscillations for three spins in a semiconductor triple quantum dot with up to 25 MHz spin rotation frequencies. This individual manipulation over three spins enables us to arbitrarily change the magnetic spin quantum number of the three spin system, and thus to operate a triple-dot device as a three-qubit system in combination with the existing technique of exchange operations among three spins.

cond-mat.mes-hall↗

arXiv2015

Single-electron Spin Resonance in a Quadruple Quantum Dot

Electron spins in semiconductor quantum dots are good candidates of quantum bits for quantum information processing. Basic operations of the qubit have been realized in recent years: initialization, manipulation of single spins, two qubit entanglement operations, and readout. Now it becomes crucial to demonstrate scalability of this architecture by conducting spin operations on a scaled up system. Here, we demonstrate single-electron spin resonance in a quadruple quantum dot. A few-electron quadruple quantum dot is formed within a magnetic field gradient created by a micro-magnet. We oscillate the wave functions of the electrons in the quantum dots by applying microwave voltages and this induces electron spin resonance. The resonance energies of the four quantum dots are slightly different because of the stray field created by the micro-magnet and therefore frequency-resolved addressable control of the electron spin resonance is possible.

cond-mat.mes-hall↗

arXiv2015

Robust micro-magnet design for fast electrical manipulations of single spins in quantum dots

Tailoring spin coupling to electric fields is central to spintronics and spin-based quantum information processing. We present an optimal micromagnet design that produces appropriate stray magnetic fields to mediate fast electrical spin manipulations in nanodevices. We quantify the practical requirements for spatial field inhomogeneity and tolerance for misalignment with spins, and propose a design scheme to improve the spin-rotation frequency (to exceed 50MHz in GaAs nanostructures). We then validate our design by experiments in separate devices. Our results will open a route to rapidly control solid-state electron spins with limited lifetimes and to study coherent spin dynamics in solids.

cond-mat.mes-hall↗

arXiv2014

Single-spin manipulation in a double quantum dot in the field of a micromagnet

The manipulation of single spins in double quantum dots by making use of the exchange interaction and a highly inhomogeneous magnetic field was discussed in [W. A. Coish and D. Loss, Phys. Rev. B 75, 161302 (2007)]. However, such large inhomogeneity is difficult to achieve through the slanting field of a micromagnet in current designs of lateral double dots. Therefore, we examine an analogous spin manipulation scheme directly applicable to realistic GaAs double dot setups. We estimate that typical gate times, realized at the singlet-triplet anticrossing induced by the inhomogeneous micromagnet field, can be a few nanoseconds. We discuss the optimization of initialization, read-out, and single-spin gates through suitable choices of detuning pulses and an improved geometry. We also examine the effect of nuclear dephasing and charge noise. The latter induces fluctuations of both detuning and tunneling amplitude. Our results suggest that this scheme is a promising approach for the realization of fast single-spin operations.

cond-mat.mes-hall↗

arXiv2014

Measurement of energy relaxation in quantum Hall edge states utilizing quantum point contacts

We demonstrated a method to probe local electronic states and energy relaxation in quantum Hall edge states utilizing quantum point contacts. We evaluated relaxation lengths in two cases; first, with electron tunneling, and second, only with energy exchange without tunneling between edge states. The results were consistent with previous experiments and validated our method with quantum point contacts. We applied this method to measure the energy relaxation length around a hotspot in quantum Hall regimes and revealed the possible short-distance relaxation process to be the relaxation due to energy exchange between edge states without electron tunneling.

cond-mat.mes-hall↗